1. Technical Field
The present invention relates to a mode locked laser device, and in particular to a mode locked laser device that outputs an ultra short pulsed light.
2. Related Art
Solid-state lasers doped with rare earth ions (or transition metal ions) have been actively developed excited by semiconductor lasers (laser diodes). Among these, ultra short pulse lasers that generate so-called ultra short pulsed light in the femtosecond range are being sought and proposed for applications across many fields such as medicine, biotechnology, instrument manufacture, measurement and the like, and through experimentation some of these lasers are starting to be applied in practice.
These ultra short pulse lasers generate ultra short light pulses by operation in so-called mode locking In simple terms mode locking is a phenomenon in which, when looking at the frequency regions when lasing, all the phases of plural longitudinal modes are synchronized (relative phase difference=0), giving rise to extremely short duration pulses due to multi-mode interference between the longitudinal modes.
Generally a Semiconductor Saturable Absorbing Mirror (SESAM) is used as one of the mirrors configuring a laser cavity (resonator), and mode locked operation occurs as a result of the increase in the steepness of the pulse in the SESAM. In addition, since the spectral band of the pulse is broad in the femtosecond region, compensation is required for positive group velocity dispersion imparted on transmission through optical materials (laser crystal, cavity mirrors and the like) in a cavity.
In particular there is a method for obtaining a pulse in the femtosecond region called soliton mode locking, in which a SESAM is disposed as a cavity mirror, mode locking is induced, resulting in a self-phase modulation effect due to the light pulse circulating in the cavity and to compensation of group velocity dispersion. Such a method is capable of self-initiation and is becoming widely employed as an excellent practical method with greater tolerance of misalignment in comparison to other methods (such as Kerr lens mode locking and the like).
In the wide definition of mode locked lasers, without restriction to soliton mode locking, large two meter-class length cavities have been reported (see, for example, FIG. 1 of Japanese Patent No. 3378103), corresponding to a pulse repetition frequency of 80 MHz.
The Pulse Repetition Frequency (PRF) is represented by PRF=C/2Lcav, where C is the speed of light and Lcav is the cavity length. When Lcav is 2 m the corresponding PRF is 75 MHz. Large ultra short pulse lasers are capable of providing an appropriate pulse repetition frequency (50 MHz to 100 MHz) and relatively high peak power (100 kW to 1 MW). However, since a mirror or the like is used for cavity folding the cavity structure is complicated, with a tendency towards an increase in the number of components and higher costs, such as an increase in manufacturing cost and the like. In addition, with large lasers there is the generally low output stability is of concern. This arises due to larger beam position fluctuations incurred from the slightest mechanical fluctuation (positional misalignment) of the cavity mirror as the size increases, resulting in output fluctuations. Periodic mirror alignment is a prerequisite for normal ultra short pulse lasers, for example, optimal mirror adjustment is required each day when the laser is operated.
Consequently, high stability ultra short pulse lasers at reduced cost are expected to arise from implementing reductions in size of lasers. By reducing the size, the component cost can be lowered by reducing the number of components, and output fluctuations due to fluctuations in the cavity length and position of the cavity mirrors can also be suppressed to the minimum. Specifically, if the cavity length is 150 mm or less, or preferably 75 mm or less, an integrated cavity structure as described below can be employed, and the stability can be raised.
By adopting a linear structure as the configuration of a cavity, optical components for folding can be omitted, and the number of components can be minimized. A solid-state laser having such a structure is described in Japanese Patent No. 3378103, for example. An optical cavity is configured here with a cavity integrated to a metal holder, with mirror installation faces set at both end faces thereof, and with a laser crystal and cavity output mirror bonded and fixed with an extremely thin layer (2 μm or less) of adhesive having a volume shrinkage ratio of 1% or less. Compactness and extremely stable output characteristics are achieved thereby. The bonding face is mirror face-polished, and layer thickness of the bonding layer is precisely controlled. According to Japanese Patent No. 3378103 the change in cavity length is 0.02 μm after operation for 5000 hours in a normal atmosphere.
Linear compact ultra short pulse lasers are described in U.S. Pat. No. 7,106,764, Japanese Patent Application Laid-Open (JP-A) Nos. 11-168252 and 2008-28379. In each case a Semiconductor Saturable Absorbing Mirror (SESAM) required for mode locking is provided as one end of the cavity mirror. By placing the SESAM and the laser crystal adjacent or in close contact to each other, and providing a cavity waist on the SESAM, more compactness can be achieved with a linear configuration, in comparison to conventional cases where cavity spots are formed separately to the SESAM and the laser crystal.
The ultra short pulse laser described in U.S. Pat. No. 7,106,764 (in particular at FIG. 15) is a configuration that realizes a mode locked laser with high repetition frequency, specifically 1 GHz and greater. The objective of this invention is to realize a high repetition frequency, however the cavity length corresponding to 1 GHz is 15 cm, and therefore realization of a cavity length of 15 cm or lower is equivalent to realization of an ultra short pulse laser with high repetition frequency. U.S. Pat. No. 7,106,764 prescribes the stimulated emission cross-sectional area of the laser medium (>0.8×10−18 cm2), the SESAM absorption depth ΔR (<0.5%), and the like.
In U.S. Pat. No. 7,106,764 a configuration is described with a linear laser cavity made from a curved mirror treated front end face of a laser crystal as an output mirror, and a SESAM disposed to the rear of the laser crystal. Configuration is also described for dispersion compensation by inducing negative dispersion by Gires-Tournois Interference (GTI) by etalon interference occurring between a SESAM and a laser crystal.
In JP-A No. 11-168252 (in particular FIG. 3) a compact ultra short pulse laser is described, with the objective of operating a ultra short pulse laser at a high repetition frequency. Specifically a liner mode locked laser is described configured with a saturable absorbing body that is coated on the rear end face of a laser crystal, and a curved chirp mirror (negative dispersion mirror).
In JP-A No. 2008-28379 (in particular FIG. 1) an extremely compact ultra short pulse laser is described, provided with a liner laser cavity configured from a negative dispersion mirror and a SESAM disposed adjacent to a laser crystal. This might be considered to be a good structure, with degrees of freedom for design and degrees of freedom for thermal interference and cavity length achieved by disposing the SESAM, negative dispersion mirror, and laser crystal separated from each other.
However, the central aim of the invention of Japanese Patent No. 3378103 is application of an integrated cavity structure to a semiconductor laser excitation solid-state laser. To be more precise, the main objective is to realize an internal cavity No. 2 high frequency laser that has continuous operation, a non-linear optical crystal disposed in the cavity, is compact, has high stability, and low cost. Consequently, there is no reference in Japanese Patent No. 3378103 to how to obtain an ultra short pulse laser that is compact, low cost and capable of high stability operation, if application were to be made to an ultra short pulse laser.
The ultra short pulse laser described in U.S. Pat. No. 7,106,764 is of semi-monolithic structure in which a laser cavity is joined to a laser crystal and SESAM, and there is no reference whatsoever to a cavity holder for disposing the laser crystal, SESAM, and the like. There is no reference in U.S. Pat. No. 7,106,764 to such matters as what sort of structure should be adopted in order to obtain an ultra short pulse laser that is compact, low cost and capable of high stability operation. In addition, while the structure described is favorable for anticipated operation with an actual repetition frequency of about 10 GHz (a monolithic structure with cavity length 1.5 cm), in contrast it would be difficult to realize a monolithic structure for 1 GHz (cavity length 15 cm) to 5 GHz (cavity length 5 cm), and application thereto is difficult.
Also, in the ultra short pulse laser described in JP-A No. 11-168252 there is a curve faced chirp mirror and laser crystal spatially separated from each other. However, there is no reference relating to a cavity holder for their support in JP-A No. 11-168252, and there is no reference to such matters as what sort of structure should be adopted in order to obtain an ultra short pulse laser that is compact, low cost and capable of high stability operation.
There is also no reference in JP-A No. 2008-28379 to a cavity holder for disposing optical components configuring the cavity separated from each other, and there is no reference therein in to such matters as what sort of structure should be adopted in order to obtain an ultra short pulse laser that is compact, low cost and capable of high stability operation.
As stated above, there is no reference made to configuration and structure of a cavity holder in the inventions related to ultra short pulse lasers described in U.S. Pat. No. 7,106,764, JP-A No. 11-168252, and JP-A No. 2008-28379, and specifically there is no reference to how implementation should be made to realize an ultra short pulse laser that is compact, low cost and capable of high stability operation. There is also no consideration relating to stability, such as to prescribed structures of cavity, temperature control of cavity length, and the like.